Positive Displacement Flowmeters

Positive displacement flowmeters measure flowrate through the continuous filling and emptying of a chamber of known volume. Positive displacement flowmeters differ from positive displacement pumps only in that, rather than being driven by motors, they are driven by the fluid flowing through them.

Positive displacement flowmeters are some of the most accurate flowmeters available today. They can easily obtain flow measurements within 0.1% of the true value. They are unique because they are the only type of flowmeter that directly measures the volumetric flowrate.

Additionally, positive displacement flowmeters require no power and are capable of handling high pressures, entrained gases, and suspended solids. The main disadvantage of positive displacement flowmeters is that they are extremely expensive to install and mantain due to their many moving parts.

Oval Gear & Impeller

General Information/Equipment Design

Oval gear meters and impeller meters, shown below, operate in the same manner, but differ in the shape of the gears. Fluid flowing through the measuring chamber causes the gears to turn, displacing an exact volume of fluid. A magnetic or mechanical device counts the number of turns, determining the volumetric flowrate.

Oval Gear

Impeller

Oval Gear

(Copyright Badger Meter, Inc., Milwaukee, WI)

Impeller

(Copyright FTI Flow Technology Inc., Tempe, AZ)

Usage Examples

The industrial oval gear meters pictured on the left are used for a variety of applications including petroleum based fluids, water solutions, and liquids compatible with the materials of construction. Pictured on the right is a handheld oval gear meter that is used in applications that require an accurate measurement of dispensed liquid volume. Examples of applications include petroleum, chemical, fuel management, lubrication, solvents and food product industries.

(Copyright Badger Meter, Inc., Milwaukee, WI)

Advantages

Disadvantages

  • High accuracy and repeatability
  • Handles viscous flow
  • Cost-effective
  • Accuracy unaffected by changes in viscosity
  • Requires little maintenance
  • Typical construction materials susceptible to corrosion by water based fluids

Helical Rotor

General Information/Equipment Design

In helical rotor meters, two nested rotors create pockets of fixed volume. The counter-rotation of the rotors carries the pockets of liquid axially down the length of the rotors. The rotation rate and number of rotations, determined by the flowrate, is measured using electronic sensors. Helical rotor meters are made of cast iron, cast steel or bronze depending on the material flowing through it.

Helical Rotor Meter

(Copyright Flowserve Corporation, Irving, TX)

Usage Examples

Common applications of the helical rotor meter involve the measurements of highly viscous materials such as concrete or molasses. Pictured below is an example of one of these meters.

(Copyright Flowserve Corporation, Irving, TX)

Advantages

Disadvantages

  • Especially well suited for measurement of highly viscous and hard to meter fluids.
  • Highest accuracy and repeatability of any positive displacement flowmeter.
  • Unaffected by flow profile and does not require straight lengths of pipe.
  • Low pressure drop.
  • Rotor action is self-cleaning.
  • Can only meter liquids.
  • Typical materials of construction will corrode in the presence of water or water-based fluids.
  • Cannot handle abrasive fluids.

Rotary Vane

General Information/Equipment Design

A rotary vane meter is composed of a circular rotor mounted inside a round compartment that contains a number of sliding vanes, which isolate fixed volumes of liquid between the rotor and the wall of the compartment. The center of the rotor is offset from that of the compartment . This keeps the rotor in constant contact with the wall of the compartment opposite that of the liquid pockets, to prevent backwashing. The more vanes in the rotor, the higher the accuracy of a rotary vane meter at low flowrates.

Usage Examples

Rotary vane meters are often used in the petroleum and agricultural industries due to their rugged construction. As with many positive displacement flowmeters, they also function as batchers .

Advantages

Disadvantages

  • Sweeping action of vanes prevents buildup of sediment and keeps compartment clean.
  • Wide choice of construction materials.
  • High accuracy and repeatability.
  • Low pressure drop over entire range of flow measuring capabilities.
  • Relatively complex design increases cost.

Rotary

General Information

Rotary positive displacement meters are similar to rotary vane meters but operate with rotors, as opposed to sliding vanes, that rotate in unison within the measuring chamber.

(Copyright Total Control Systems, Fort Wayne, IN)

Equipment Design

As fluid flows into the chamber, the blocking rotor is forced to rotate and the movement of the fluid is translated from linear motion to rotational motion. The displacement meters rotate with the blocking meter to guide the fluid though the chamber. The blocking meter's rotation determines the output of the meter and is transmitted to a register.

(Copyright Total Control Systems, Fort Wayne, IN)

There is no metal-to-metal contact between the rotors and housing walls, which prolongs the life of the flowmeter.

Pictured below are two commonly used rotary flowmeters.

(Copyright Total Control Systems, Fort Wayne, IN)

Usage Examples

Rotary flowmeters are used for the measurement of a variety of fluids including gasoline, oils, liquefied petroleum gas, industrial chemicals, solvents, corn syrup, and adhesives. Pictured below is a rotary flowmeter used to meter refined petroleum products.

(Copyright Total Control Systems, Fort Wayne, IN)

Reciprocating Piston

General Information/Equipment Design

Fluid enters a reciprocating piston meter, also known as an oscillating piston meter, through an inlet port. The port leads to a precisely measured gap created by a piston in a round chamber. As the piston oscillates around the chamber, it displaces an exact volume of fluid.

Electromagnetic sensors outside the meter detect the rotation of a magnet inside the center of the piston, which indicates the quantity or rate of fluid passing through the meter. Below is a picture of a reciprocating piston meter.

(Copyright Total Control Systems, Fort Wayne, IN)

Usage Examples

The piston meter to the left is part of a system that handles diesel exhaust fluid. In the middle picture, a piston meter is used in renewable fuel applications. Piston meters are good for low volume measurement applications such as fueling light aircraft and helicopters, shown on the right. Other industries that use these flowmeters include chemical, pharmaceutical, petrochemical, and food and beverage.

(Copyright Total Control Systems, Fort Wayne, IN)

Advantages

Disadvantages

  • High accuracy and repeatability.
  • Only one moving part to cause wear.
  • Can be made of materials to ensure sanitary needs of food and beverage processing.
  • Can only be used with relatively clean liquids.

Nutating Disc

General Information/Equipment Design

A disc attached to a sphere is mounted inside a spherical chamber. As fluid flows through the chamber, the disc and sphere unit nutates . The nutation causes a pin, mounted on the sphere perpendicular to the disc, to rock. Each revolution of the pin indicates a fixed volume of liquid has passed. A mechanical or electromagnetic sensor detects the rocking of the pin.

(Copyright Badger Meter, Inc., Milwaukee, WI)

Usage Examples

The nutating disc meter below is manufactured to handle hot water up to 250°F. It has a liquid-crystal polymer disc and a bronze housing. Disc meters made with aluminum or bronze discs are commonly used to meter hot oil and chemicals.

(Copyright Badger Meter, Inc., Milwaukee, WI)

Advantages

Disadvantages

  • May be constructed from a variety of materials.
  • High accuracy and repeatability.
  • Accuracy is adversely affected by viscosities below the meter's designated threshold.

Acknowledgements

Badger Meter, Inc. , Milwaukee, WI

Flowserve Corporation , Irving, TX

FTI Flow Technology Inc. , Tempe, AZ

Total Control Systems , Fort Wayne, IN

References

Digiacomo, Ronald W., "Measuring Flow." Chemical Engineering , 7(2011): 30 - 34.

"Engineering Manual: 700 Series Rotary Flow Meter." Total Control Systems . Total Control      Systems, 2007. Web. 10 July 2011. <http://www.tcsmeters.com/pdf/700/tcs900005.pdf>.

McCabe, Warren L., Smith, Julian C. and Peter Harriot. Unit Operations of Chemical Engineering . New York: McGraw-Hill, 1993: 227. Print.

Omega Complete Flow and Level Measurement Handbook and Encyclopedia, Vol. 29. USA:      Omega Engineering Inc., 1995: Z-8, Z-11 - Z-18. Print.

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Erica Mauter

Steve Wesorick

Kelsey Kaplan

Andrea Roberts

Henry Chen

Fritz Hyde